just for fun

I'm re-creating the classic PacMan for the fun of it and needed a way to translate back and forth between directions and dx, dy for a sprite. Here was a fun solution that avoided cumbersome if...elif...elif...elif... chains. Any thoughts on readability, efficiency? Specifically, if this gets called by 4 monsters and 1 pacman every clock-tick = 1/60 sec, am I likely to run into trouble?